1. Field of the Invention
This invention relates to an apparatus for use in a monocrystal production method in accordance with the Czochralski technique, adapted for measuring the diameter of a crystalline rod at the interface between the crystalline rod and a melt.
2. Description of he Prior Art
In a known crystal diameter measuring apparatus of this type, as shown in FIG. 4, an imaginary straight line which extends across a luminous ring 14 detected by its high luminance at the interface between a monocrystal rod 10 and a melt 12 and passes through the center of the luminous ring 14 is used as a sensing line of an image sensor, and positions of the picture elements that correspond to positions Q.sub.11 and Q.sub.12 on the luminous ring 14 are detected so as to obtain the distance between these positions, thereby measuring a diameter D of the monocrystal rod 10.
However, if, as shown in FIG. 5, the level of the melt 12 drops from a level H.sub.1 to a level H.sub.2 as the monocrystal rod 10 grows, the light emitted from a position Q.sub.3 at the rear of the monocrystal rod 10 can not strike the image sensor 16, so that it becomes impossible to detect the luminous ring 14. This makes it necessary to move downward the one-dimensional image sensor 16, which consists of picture elements disposed on a straight line, or rotate it counterclockwise by a predetermined angle so that the sensing line pssses through a position Q.sub.21 vertically below the position Q.sub.11. Accordingly, the provision of a complicate mechanism for vertically moving or rotating the one-dimensional image sensor 16 is needed. Moreover, the operation of finely adjusting the position is not an easy task.
If, in order to avoid this problem, the crucible that contains the melt 12 is moved upward so that the level H is kept constant and the one-dimensional image sensor 16 is independently at a fixed position, the relationship between the positions of the melt 12 and the heater used for heating the melt 12 deviates from that which gives the optimum growth conditions, resulting in a reduction in the quality of the crystal.
In addition, it is necessary in order to secure the detection of the luminous ring 14 even if the monocrystal rod 10 wobbles slightly that, in practice, the positions Q.sub.11 and Q.sub.21 shown in FIG. 5 must be located slightly forward on the luminous ring 14 despite of a reduction in the accuracy of measurement of the diameter D.